The contents of a device class are determined by whether the device accesses the data on its media randomly or sequentially.
Devices that access their media randomly share a common ADSM device type. ADSM provides a single, random-access device class, named DISK. You cannot define other random access device classes. You do not define an ADSM library for random access devices.
Random access device types store data in blocks of storage that can be scattered across the available space on a disk. As the server deletes data that has expired, the space occupied by that data can be reused.
Tape devices, optical disk devices, FILE device types, and SERVER device types are members of the sequential access category of devices. All of these devices access their data sequentially. A device class for a sequential device contains a device type and media management information.
For tape and optical sequential devices, the device class also specifies a library. Figure 6 shows the contents of a device class for a typical sequential access device.
Figure 6. Contents of a Device Class for Sequential Access Devices
Sequential access device types begin to store data at the beginning of a volume, and append new data after existing data. As data is deleted, the space is not immediately reused. The server can reclaim space later by using the reclamation process (see "Reclaiming Space in Sequential Access Storage Pools" for details).
Every sequential access device class requires one of the ADSM device types as part of its definition. A device type identifies a device as a member of a group of devices that share similar media characteristics. ADSM provides device types for many devices. For example, ADSM provides a DISK device type for disk drives and an 8MM device type for tape drives that use 8mm tape.
FILE is an ADSM sequential device type that allows the administrator to create sequential volumes by creating files on disk storage. To the ADSM server, these files have the characteristics of a tape volume.
The sequential device type called SERVER allows the server to store data on another ADSM server. This data has the characteristics of a tape volume from the view of the source server. On the target server, this data appears as archived files that belong to a special type of node.
For sequential access device types (excluding FILE and SERVER), you must specify a library in the device class definition. The library you specify must be one that you have defined to ADSM, as discussed in "Library".
Every sequential access device class contains media management information, such as recording format and labeling prefixes.
For more information about how ADSM helps to manage media, see the following: